London’s airspace has been badly affected by a computer failure at the main air traffic control centre this afternoon.

The National Air Traffic Service (NATS) has reported (at 4.15pm) that the system has been restored and it was 'in the process of returning to normal operations'.

Earlier it said capacity had been cut until things were sorted out.

"NATS can confirm that a technical problem has been reported at Swanwick air traffic control centre," it said in a statement. "UK airspace has not been closed, but airspace capacity has been restricted in order to manage the situation. We apologise for any delays and our incident response team has been mobilised. Every possible action is being taken to assist in resolving the situation and to confirm the details."

Heathrow Airport is expected to become increasingly chaotic tonight as passengers are turned away from flights they expected to board and the terminals begin to fill up. The closure is expected to be in place until at least 7pm.

A Heathrow Airport spokesman said flights had been affected since just after 3pm. He said no flights were being allowed to take off from the airport but planes were still landing.

"We understand it is affecting UK airspace," he said. "There are a few [planes] in the air that are coming in that are being dealt with and landing but nothing is taking off at the moment. We hope to know more in due course."